1. Field of the Invention
The invention relates generally to the field of digital-to-analog conversion, and particularly to digital-to-analog conversion in radio transmitters.
2. Background Information
Digital-to-Analog Converters (DACs) are often found in signal generation applications such as radio transmitters. See for example U.S. Pat. No. 6,032,028 and U.S. Pat. No. 5,010,585.
As shown in FIGS. 1A-1D, the signal at the analog output of a DAC contains not only the desired signal, but also alias signals (or spectral copies) which have to be removed in a reconstruction filter, as shown for example in FIGS. 1A-1D.
As shown in FIG. 1A, a digital input signal is provided to an input 102 of the DAC 104. An output 106 from the DAC 104 provides an analog signal to an input of a reconstruction filter 108, and the reconstruction filter 108 outputs a reconstructed analog signal on its output 110. FIG. 1B shows a magnitude spectrum of the digital signal provided to the input 102 of the DAC 104.
The spectral copies in the output from the DAC 104, fall off in amplitude as (sin(x))/x due to the zero-order hold (ZOH) of the DAC. This can be seen in FIG. 1C, which shows a magnitude spectrum of the signal output from the DAC 104. The sinc response 142 behavior of the magnitude spectrum of the signal, which is due to the ZOH of the DAC, can be seen in FIG. 1C. FIG. 1C also shows an ideal frequency response 140 of the reconstruction filter 108. FIG. 1D shows a magnitude spectrum of the signal output in the transmit frequency spectrum from the reconstruction filter 110, when the reconstruction filter 110 operates in accordance with the ideal frequency response 142.
For performance reasons, the DAC outputs a desired signal at a fairly low frequency. Consequently, the reconstruction filter 108 is often implemented using discrete LC technology. If high filter performance is required, the filter will often be bulky and require trimming.
In a typical radio transmitter known in the prior art, the output signal from the DAC is frequency upconverted in a mixer after passing through the reconstruction filter. An example of this is shown in FIG. 2, where an output of the reconstruction filter 108 is provided to a mixer 212. The mixer 212 mixes the output from the reconstruction filter 108 with a signal from a local oscillator 214. The frequency of the signal from the local oscillator 214 is represented as fLO. The output of the mixer 212 is provided to an IF filter 216.
In typical frequency mixers, the input signal is not only frequency-translated with the local oscillator frequency fLO, but is also frequency-translated with its odd harmonics. Any of the spectral copies that survive the reconstruction filter 110, or that are present in circuit configurations where the signal from the DAC 104 is provided directly to the mixer 212 (i.e., no reconstruction filtering is performed) can cause unacceptable distortion in or close to the signal band of interest. The distortion occurs because the spectral copies mix with the fundamental or harmonic frequencies of the local oscillator 214. FIGS. 3A, 3B show an example of this distortion.
FIG. 3A shows a magnitude spectrum of a signal provided from the DAC 104 directly to the mixer 212 (where no reconstruction filtering is performed). FIG. 3B shows a magnitude spectrum of an output signal from the mixer 212, which results when the signal of FIG. 3A and the signal from the local oscillator 214 are input to the mixer 212. For the sake of illustration, the harmonics of the local oscillator 214 are assumed to have the same magnitude level as the fundamental frequency fLO of the local oscillator 214.
If the mixer 212 is well balanced, then only odd-order harmonics are of practical importance. These odd-order harmonics fall off in amplitude approximately following a Fourier series expansion of a square wave. The spectrum of the output of the mixer is simply found by displacing the double-sided spectrum of the output signal from the DAC, by all possible integer multiples (positive and negative) of the local oscillator frequency fLO.
For these reasons, the reconstruction filter 108 needs to be placed before the mixer 212. The filter requirements will become dependent on the frequency plan of the transmitter. In narrowband transmitters, its is possible through careful frequency planning to prevent frequency-converted spectral copies from appearing inside the frequency band of interest. For transmitters having bands wider than narrowband transmitters, effective frequency plans are very difficult to achieve due to the larger bandwidth of the signal.
Moreover, high performance DACs usually have current source output, and suffer from decreasing linearity as the voltage swing over the load of the DAC increases. It is hence beneficial to keep the load impedance as low as possible, in order to help the DAC perform in as linear a fashion as possible. On the other hand, the linearity of some types of mixers is enhanced or benefitted by driving the mixer with a current source (i.e., high impedance source). However, it is not possible to provide both a low impedance load for the DAC, and a high impedance source for the mixer, without signal attenuation which in turn degrades the attainable signal-to-noise ratio (SNR). In summary, the following problems exist in prior art systems: 1) a bulky and often expensive reconstruction filter is required in front of the mixer, and 2) the reconstruction filter cannot present both an optimal load for the DAC, and an optimal source for the mixer, without causing signal loss. Accordingly, a need exists for a system to overcome these problems.
In accordance with exemplary embodiments of the invention, a circuit is presented that eliminates the need for a bulky filter between the DAC and the mixer, and provides both a very low load impedance to the DAC, and a high source impedance to the mixer. In accordance with an exemplary embodiment of the invention, the DAC is connected directly to the mixer, and an IF filter after the frequency upconversion (i.e., located downstream of the mixer) is used as a reconstruction filter. In addition to eliminating the bulky prior art reconstruction filter between the DAC and the mixer, a direct connection between the DAC and the mixer provides the DAC with a low-impedance load, and provides the mixer with a high-impedance source because the DAC and the mixer are effectively current-matched. In accordance with an embodiment of the invention, the update rate of the DAC, fDAC, is related to the frequency of the local oscillator by the following relation: fLO=(n/2)(fDAC), where n is any positive integer. In an exemplary embodiment of the invention, n=1 so that fLO=(fDAC)/2.